Gas barrier resins such as ethylene-vinyl alcohol copolymer (hereinafter sometimes abbreviated as EVOH) are materials having excellent oxygen gas barrier properties and carbon dioxide gas barrier properties. Such a resin can be melt-molded and therefore is used preferably for a multilayered plastic packaging material having a layer of the resin laminated with a layer made of a thermoplastic resin (polyolefin, polyester, etc.) having excellent moisture-resistance, mechanical properties and the like. However, the gas permeation through such gas barrier resins is not completely zero, and such gas barrier resins transmit an amount of gas that cannot be ignored. It is known to use an oxygen absorbent in order to reduce transmission of such a gas, in particular, oxygen, which significantly affects the quality of the content, or in order to remove oxygen that is already present inside a package at the time of packaging its content.
For example, as an improved oxygen absorbent, a composition containing a transition metal catalyst and an ethylenically unsaturated compound has been proposed (see Japanese Laid-Open Patent Publication No. 5-115776). Furthermore, resin compositions containing EVOH as described above and an oxygen absorbent have been proposed (Japanese Laid-Open Patent Publication Nos. 2001-106866, 2001-106920 and 2002-146217). In particular, similar to EVOH, the resin compositions containing EVOH described above can be melt-molded and therefore can be for use preferably for various packaging materials.
However, when an oxygen absorbent or an oxygen-absorbing resin composition as mentioned above is used as a packaging material, the oxygen absorbent is decomposed as oxygen absorption proceeds, and an unpleasant odor may be generated. Therefore, there are demands for a further improvement for applications in which fragrance is important. The inventors conducted extensive research to address the problem described above, and as a result, have arrived at the invention of an oxygen-absorbing resin that does not generate an unpleasant odor (see Japanese Laid-Open Patent Publication No. 2005-187808).
However, when the contents are stored for a long period of time, a packaging material is desirable that an oxygen absorption amount of the packaging material is as more as possible, and therefore, a further enhancement of the oxygen absorbency of oxygen-absorbing materials is required without generating the above-described unpleasant odor. For that purpose, for example, it can be considered to increase portions to be oxidized in an oxygen-absorbing material. That is, it can be considered to increase, by increasing double bonds in the material, the amount of allylic positions (methylene or methine carbon adjacent to a double bond) that are considered as portions to be oxidized which are relatively highly reactive. However, materials having many double bonds therein are problematic in being inferior in stability and processability during melt-molding and being likely to be colored or generate aggregation during molding. Therefore, it is not sufficient just to increase double bonds, and there is a limit to the concentration of double bond in a material. In molding processing, it is also required that adhesion of a resin to an extrusion screw or the like should not occur, i.e., that the handling properties during processing is excellent. Moreover, in the field of food packaging, when packaging is carried out using a film containing an oxygen-absorbing resin in the base resin, immediate removal of oxygen remaining inside a package may be required to further improve the shelf life of a packaged food. In this case, it is required to attain not just much oxygen absorption amount, but a high oxygen absorption rate within a short period of time during the initial stage.
As such a method to increase the oxygen absorption rate, it can be considered to improve the dispersion of the oxygen-absorbing resin contained in the base resin. However, even when the dispersion is increased by, for example, adding a compatibilizer so as to reduce the average particle size, the rate of absorption is not always increased so much.
Furthermore, for food packaging materials, materials having further excellent low odor, high safety and increased transparency are required as compared with conventional products. In particular, great demands exist in recent years for retort packaging materials, and thus materials that do not allow an odorant or the like to be dissolved in or transferred to aqueous food, beverages, etc., under severe conditions such as those in retort processing are required.
As described above, there is a demand for an oxygen-absorbing resin composition having extremely high oxygen absorbency that neither generates an unpleasant odor nor causes the various problems described above.